In a stepping motor of a conventional electronic timepiece, a stepping motor driving means has been used in which a main driving pulse train having low effective power is output to the stepping motor to reduce current-consumption, and wherein the rotational state of a rotor is detected by known means to output a correction drive pulse to the stepping motor in accordance with the detection result. Practical examples of this is known technology are disclosed in Japanese publication of applications No. 61-8392 and No. 63-18148, for example.
With respect to the practical example illustrated in Japanese publication of application No. 61-8392, FIG. 2 diagrammatically shows a drive voltage waveform of a correction drive system, and FIG. 3 diagrammatically shows a voltage waveform after turning off a main drive pulse obtained by the drive system as shown in FIG. 2.
The drive voltage waveform diagram as shown in FIG. 2 is constructed by a main drive pulse P1 (hereinafter referred to as "P1") which is output to the stepping motor every second, a section DT for detecting the rotation of the stepping motor after P1, goes low and a correction drive pulse P2 (hereinafter referred to as "P2") which is output when the stepping motor is in non-rotational state. P1 automatically alters its pulse width in accordance with a load state to be applied to the stepping motor. P2 is output
FIG. 3 shows a voltage waveform which is induced in a detection resistor by forming a closed loop in a coil after pulse turning off through the control of a MOS gate for driving the stepping motor or the like. As shown in FIG. 3, a rotation detecting method is used for detecting rotation of the rotor by electrically detecting whether the induced level reaches a predetermined voltage on the basis of the fact that the induced voltage in the section DT is different between a rotational state (as indicated by a solid line in FIG. 3) and a non-rotational state (as indicated by a dotted line in FIG. 3).
This detection means is characterized in that the rotor which is rotated with the main drive pulse makes free attenuating motion due to residual magnetic potential energy in the rotor after the main drive pulse goes low, and the variation of the induced voltage occurring in the coil during the attenuating motion is used as the rotation detecting means.
With respect to the practical example which is illustrated in Japanese publication of application No. 63-18148, FIG. 4 shows an example of a drive voltage waveform diagram of a correction drive system, and FIG. 5 shows an example of a current waveform generated when the rotor is driven by the detection pulse.
The driving voltage waveform diagram of FIG. 4 is constructed by a main drive pulse P1 which is output to the stepping motor every second, detection pulses Px and Py which are used to detect the rotation of the stepping motor after P1 goes low, and a correction drive pulse P2 which is output when the stepping motor is in a non-rotational state at P1. P1 and P2 are identical to those of FIG. 2. The detection pulses Px, Py have such a short pulse width that the stepping motor cannot be rotated.
FIG. 5 shows the current waveform when the rotor is driven with the detection pulse, and the current waveform is varied as shown by a characteristic curve a or characteristic curve b of FIG. 5 in accordance with the orientation of the magnetic pole of the rotor. The reason for the difference in current waveform is that the current waveform is determined in accordance with whether the magnetic pole formed in a stator with detection pulse is in a state where the magnetic pole of the rotor magnet has a repulsive orientation or an attractive orientation. As shown by curve b of FIG. 5, the rotation detecting means for the rotor drives the rotor with the detection pulse, and identifies the orientation of the magnetic pole of the rotor on the basis of the difference in shape of the current waveform flowing in the coil at the driving time of the rotor, thereby to detect the rotation of the rotor.
The detecting means is characterized in that the rise-up voltage of the current waveform (the rise-up shape of the voltage waveform) is detected using the detection pulse having such effective power as to reduce the current consumption to detect the position of the magnetic pole of the rotor magnet, whereby the rotation of the rotor is detected.
However, the conventional rotation detecting method has the following problem for accurate judgment of the rotation of the rotor.
An induced voltage which is caused by the attenuating motion of the rotor within a predetermined time (for example, a period from the start of pulse application to 8-16 msec) has the relationship between the induced voltage and the pulse width of the main drive pulse as shown in FIG. 6 and the relationship between the induced voltage and the moment of inertia of the rotor as shown in FIG. 6.
The relationship between the main drive pulse and the pulse width is explained with the solid of FIG. 6 showing the induced voltage waveform.
When the main drive pulse is applied to the stepping motor, the induced voltage is a sufficiently high voltage in a range of the shortest pulse width T1 to a degree of long pulse width T2 where a normal stepping operation can be performed. However, when the pulse width exceeds T2, the induced voltage is rapidly lowered. This phenomenon is due to the following reason: the rotor has low magnetic potential energy after a pulse having a long pulse width is turned off, and the amplitude of the free attenuating motion of the rotor is reduced, so that the induced voltage is lowered in proportion with the amplitude of the attenuating motion of the rotor.
The relationship between the induced voltage and the inertia moment of the rotor is explained with the solid curve of FIG. 6 showing the induced voltage waveform. A stepping motor having a rotor of low inertia moment can not only reduce power consumption but can also more easily and rapidly rotate and stop. That is, the rotor of low inertia moment can be rotated with a small amount of effective power, and stopped in a short time after turning off a pulse through an attenuating motion of small amplitude. When the amplitude is small as described above, the absolute value of magnetic flux which intersects the coil is small, and the induced voltage caused by the attenuating motion of the rotor is also lowered. Further, since the free attenuating motion of the rotor is rapidly reduced, if a method of detecting the induced voltage in a predetermined time is used, the rotor almost approaches to a stopped state. Accordingly, there exists virtually no induced voltage which is caused by variation of magnetic flux per unit time, and the rotation state of the rotor is erroneously judged to be at a non-rotational state.
Accordingly, in the rotation detection method of the rotor using the induced voltage occurring after the application of the main drive pulse, if the power consumption is increased in order to lengthen the pulse width of the main drive pulse to improve a driving torque and the inertia moment of the rotor is reduced to lower the power consumption, the amplitude of the rotation free attenuating motion of the rotor after the turning off the pulse is lowered to induce the lowering of the induced voltage, resulting in the erroneous judgment of the rotation of the rotor.
Further, in the detection method using the detection pulse, the pulse width of the detection pulse must be designed to be long to some extent to enable accurate judgment of the magnetic pole of the rotor, and thus there occurs a problem that the current consumption of the stepping motor is increased.
Still further, the judgment on the rotation of the rotor is erroneously made if the detection pulse is output from a rest state of the rotor, and thus the output timing of the detection pulse must be delayed. Accordingly, the output timing of the correction drive pulse which is output in the non-rotational state of the rotor is delayed, so that the motion of an indicator is delayed and provides an unnatural display.